DUI Defenses – Ignorance, Margin of Error & Partition Ratio

With some criminal laws, not knowing what you are doing, or ignorance, is a defense to being punished for the crime. This is not true for DUI-DWI offenses. No good case law exists in any jurisdiction for such a defense to impaired driving. However, a person may NOT intend (or even know) that he or she is driving, if certain prescribed medication is taken and the person begins “sleep driving.” This rare and somewhat difficult defense is based on “lack of intent to drive.”

Your attorney should always inquire about any medications or pills that you may have taken prior to driving.

DUI Defense – Margin of Error Variance

Some breath test programs euphemistically call breath machine error “sampling variability.” With any scientific or electronic test, a margin of error in the machine being able to precisely measure a sample will always exist.

In most states, the acceptable “difference” between two breath exhalations is 0.020 or less between the two samples. If the breath test results are “borderline,” then your attorney may be able to present the defense to the “per se” DUI-DWI charge that the State cannot prove beyond a reasonable doubt that your breath alcohol level was actually at or above the requisite alcohol level.

Usually, an expert witness is needed to assert this defense, as well as thorough discovery about the reliability of the State’s calibration checks on the particular breath machine.

DUI Defense – Partition Ratio

The basic breath machine is set up to “estimate” a blood alcohol level based on a standard ratio of breath to blood of 2100 units of breath to 1 unit of blood in your body.

A DUI partition ratio defense can be raised as a legitimate, scientific attack if your ratio is always lower (e.g., 1400 to 1) and this can be documented. Again, to establish the “partition ratio” defense requires the use of one or more expert witnesses. Most experienced DUI-DWI trial attorneys will only assert this attack if no better explanation of an errant breath test result is available to your defense lawyer.

In at least a few states, this defense has been legislatively eliminated or determined to be irrelevant and upheld by appellate courts in that state.